It's easy to change your attendance if you have the app. If you enter the event and press the button that says 'Attending' or 'Decline', you can toggle your response at any time.

To see past or declined events, press Upcoming in the event overview to change the filter. You can also decide if you want to show repeating and declined events in the overview.

Those who reply through email notifications can change their response in the desktop user interface using the same link they received when they were invited to the event. If you register and login on, you'll be able to get a full overview over your events.
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